- Fan stalling (fatigue damage due to stalling accumulates with repeated or sustained stall and can result in catastrophic fan failure).
-
- Fan performance other than that anticipated when the fan was initially selected (less flow, higher pressure, greater power consumption due to reduced fan efficiency).
Reducing or increasing the fan’s rotational speed respectively reduces or increases the height of the fan’s characteristic curve (as exhibited on the example curves below). The fan efficiency contours move in coincidence with the individual curve. With that, optimum efficiency is maintained across a broadened range of duty points.
Air flow varies directly as the change in speed. That is:
(RPM2 / RPM1) Air Volume 1 = Air Volume 2
Ex.: (1800 RPM/900 RPM) 100,000 cfm = 200,000 cfm
Pressure varies as the square of the speed change ratio. That is:
(RPM2 / RPM1)2 Pressure 1 = Pressure 2
Ex.: (1800 RPM/900 RPM)2 4-in. w.g. = 16-in. w.g.
Power (HP) varies as the cube of the speed change ratio. That is:
RPM2 / RPM1)3 HP 1 = HP 2
Ex.: (1800 RPM/900 RPM)3 63 HP = 504 Air HP (exclusive of fan efficiency)
Reducing or increasing the number of fan blades (altering solidity) respectively reduces or increases the height of the fan’s characteristic curve (as exhibited on the example curves below). The fan efficiency contours move in coincidence with the individual curve. With that, optimum efficiency is maintained across a broadened range of duty points.
- Varying the rotational speed of the fan impeller (rotor) – Example Curves Below
- Varying the blade angle (axial flow fans). – Exhibited on the example curves below.
- Varying the number of fan blades (i.e. "half- blading" axial flow fans) – Example Curves Below
- Employing an inlet control device such as inlet vane control (common to centrifugal fans, but not generally recommended for axial flow fans)